Return errno when fopen fails on request_fs_file_channel_open

[jvazquez-r7]

On Windows environments request_fs_file_channel_open returns the GetLastError result code when fopen fails. But, on linux environment, there isn't GetLastError, and request_fs_file_channel_open fails to return the errno code. Because of that, the (client) PacketDispatcher fails to find an error on the response, not handling the error, which ends with unhandled exceptions. It's easy to reproduce by trying to update a file to a nonexisting remote directory on linux meterpreter:

[*] Started reverse handler on 172.16.158.1:4444
[*] Starting the payload handler...
[*] Transmitting intermediate stager for over-sized stage...(100 bytes)
[*] Sending stage (1228800 bytes) to 172.16.158.188
[*] Meterpreter session 1 opened (172.16.158.1:4444 -> 172.16.158.188:57584) at 2014-07-08 12:01:01 -0500

meterpreter > upload /tmp/pwned.txt /nonexistent
[*] uploading  : /tmp/pwned.txt -> /nonexistent
[-] Error running command upload: NoMethodError undefined method `value' for nil:NilClass
meterpreter >

By turning errno on non-winidows environments, the meterpreter PacketDispatcher is able to catch the error, and unhandled exceptions not happen anymore:

[*] Transmitting intermediate stager for over-sized stage...(100 bytes)
[*] Sending stage (1138688 bytes) to 172.16.158.188
[*] Meterpreter session 1 opened (172.16.158.1:4444 -> 172.16.158.188:57586) at 2014-07-08 12:03:20 -0500

meterpreter >
meterpreter >
meterpreter > upload /tmp/pwned.txt /nonexistent
[*] uploading  : /tmp/pwned.txt -> /nonexistent
[-] core_channel_open: Operation failed: 2

On the other hand, the linux meterpreter (client) doesn't solve errno codes atm, showing just the error identifier. When handling with windows machines, the meterpreter (client) is able to handle GetLastError() codes, showing more user-friendly errors. It's on my TODO, but out of scope change for this pull request, which just tries to solve unhandled exceptions when dealing with request_fs_file_channel_open.

[metasploit-public-bot]

Test PASSED.
Refer to this link for build results: https://ci.metasploit.com/job/GPR-MeterpreterWin/119/

[OJ]

I think #3505 is the one.

Sorry for not getting onto this fast enough guys.

[Meatballs1]

Yarp I saw in the reference bit as the POST request went out ;)